1. Field of the Invention
The invention relates generally to respiration apparatus for administering oxygen to a patient, and in particular to a positive pressure, demand responsive fluidic controller.
2. Description of the Prior Art
Positive pressure respiration systems are used in inhalation therapy for the administration of oxygen and adjuvant gases in the treatment of patients having pulmonary disorders such as asthma or pneumonia. Most conventional respirators are fairly complex and employ a variety of elements including snap valves, springs, solenoid valves, magnets, gear boxes, ratchets, mechanical linkages, pulleys, photocells, electronic circuitry, and other components to provide a number of functions. The complexity and hybrid nature of these systems results in large, heavy and expensive units which are somewhat fragile and liable to mechanical failure.
Positive pressure respiration systems normally fall within two broad classifications, that is, positive pressure volume limited systems and positive pressure flow cut-off or flow limited systems. Positive pressure volume limited respiration systems are distinguishable by the fact that in the use thereof, a predetermined volume of gas is forced into the patient's lungs at predetermined intervals. In positive pressure flow cutoff respiration systems, the patient initiates the flow of gas by slight inspiration, after which the patient is forced to inspire as much of a desired gas, such as oxygen, as his lungs will permit or accept.
In yet another category of positive flow respiration devices, oxygen is continuously delivered at a substantially constant, regulated flow rate to a nasal cannula for inspiration by a patient. In this arrangement oxygen is continuously delivered to the patient at a constant rate during expiration as well as during inspiration. Since the respiration cycle is approximately 30% inspiration and 70% expiration, a substantial amount of oxygen is vented without recovery during expiration. Because the nasal passages are being ventilated constantly, the oxygen must usually be circulated through a humidifier to improve the patient's comfort during the extended administration. These humidifiers represent potent sources of nosocomial (hospital acquired) infections. Additionally, these humidifiers require much staff attention in keeping the water level up and changing systems 2-4 times per day. Furthermore, the accumulation of a large volume of vented oxygen around a patient's bed, especially within a small room, represents a fire safety hazard. Moreover, since the cost of oxygen administration is directly related to the total amount of oxygen delivered to the patient, including that volume of oxygen which is vented rather than being inspired, a substantial percentage of the cost of the oxygen administration service could be saved if some way could be found to substantially reduce or eliminate the venting of oxygen during expiration.